Rotary multicolor printing presses to which the invention is directed have a plurality of printing stations or printing units and a central drive gear which is common to all the printing units for operation thereof.
Machines of this kind may be, for example, the printing presses known as flexo or flexographic printing presses. Such presses are commonly used for high speed printing on material in web form. The web to which the printed image is to be applied is passed in the respective printing unit between a plate cylinder thereof and a cooperating impression cylinder. The plate cylinder is arranged for linear contact with the web to minimize the possibility of smearing the image. The fact that the press has a plurality of printing units permits a plurality of inks of different colors to be applied to the web in successive printing operations. Arrangements for drying the printing ink which is applied in a given printing unit may be disposed downstream of that unit for drying the ink which has just been applied, so that a plurality of such drying devices are disposed between respective adjacent printing units. The web may range from very absorbent materials having relatively low image registration accuracy requirements, such as textile fabrics, to smooth, thin plastic films and metal foils having virtually no ink absorbency and very high image registration requirements arising both from the difficulties of handling and printing on such materials and the detail of the images often required to be printed on such materials.
In one common form of such presses, each of the printing units includes an inking mechanism, a plate cylinder and an individual impression cylinder cooperating with only the plate cylinder of the printing unit. Typically, each plate cylinder of the respective printing units is driven by the above-mentioned central drive gear by way of a further gear which is generally non-rotatably connected to the respective plate cylinder. The drive for the respectively associated impression cylinder in each printing unit may be derived from the drive for the plate cylinder or directly from the central drive gear. By gear driving the plate cylinders directly from the central drive gear, the sources of play between the central drive gear and each plate cylinder, and thus misregistration (angular misalignment) among the plate cylinders are minimized.
In another common form of such rotary multicolor printing press, a single central impression cylinder of a correspondingly large diameter is provided cooperating with all of the plate cylinders. In such presses, the plate cylinders of the individual printing units are arranged at the periphery of the single central impression cylinder. Such a printing press design is intended to take account of the fact that, when printing very thin material, for example films or foils from which carrier bags are to be produced, such material is frequently subject to considerable stretch effects when using the conventional printing units in which each plate cylinder has associated therewith its own respective impression cylinder. By using a large impression cylinder, as indicated above, there is less stretching of the material occurring, due to the large contact surface area between the impression cylinder and the web of material to be printed. Consequently, the web is moved with a high degree of accuracy and minimal unwanted stretch through and between the print stations. Generally speaking, this provides for more accurately registered individual images and thus a sharper overall multicolor image than can be achieved with presses employing separate impression cylinders associated with each print station. However, the large single impression cylinder entails a disadvantage, namely expense. A large central impression cylinder is extremely expensive to manufacture, especially as the peripheral surface thereof must be machined to a very high degree of accuracy and the diameter thereof must be kept constant also with a high degree of accuracy.
British laid-open application GB 2 111 912 (corresponding to German laid-open application (DE-OS) No. 31 50 833, now Patentschrift No. 3 150 883) discloses a flexographic rotary multicolor press with cooperating pairs of plate and impression cylinders which can be adjusted to accommodate print format changes, i.e., changes in the plate cylinder diameters. The press includes six printing units each with an impression cylinder supported by vertical frame members within the circumference of a main or central drive gear. Each printing unit further includes a plate cylinder supported with inking cylinders on lands arranged around the central drive gear. Each of the printing mechanisms of this press include a gear connected to the plate cylinder, a gear connected to the impression cylinder, and the two gears directly engaged with the central drive gear. Whenever the diameter of the plate cylinders is altered, it is necessary for the plate cylinders to be repositioned, due to the changes in the spacings between the axes of rotation thereof as a result of the changes in the diameters of the plate cylinders, in order to keep the plate cylinder gears engaged with the central drive gear. In addition, the impression cylinders also have to be repositioned, at least in some of the printing units or stations, to keep the plate and impression cylinder pairs in diametric alignment with the central gear to prevent wrap of the web around the plate cylinder. Thus, format changes involve radial and often tangential movement of the plate cylinders and often tangential movement of the respective impression cylinders. That means that the operation of converting the machine to different formats to be printed becomes a complicated operation. Furthermore, when the position of both the plate cylinders and also the respectively cooperating impression cylinders are changed, the length of the portion of material to be printed, between two adjacent printing units, also changes with the result that further additional modifying steps have to be taken when setting the printing press after a change in format.
From the foregoing, it can be seen that it is desirable to provide as high a degree of accuracy in registration as is economically feasible in high speed rotary multicolor presses employing separate impression cylinders to be able to compete favorably in terms of quality with more expensive presses employing a single central impression cylinder.
It is further desirable to provide in such presses the ability to simply and rapidly convert between different print formats while maintaining a high degree of accuracy of registration.